Field of the Invention
The present invention relates to the field of the vehicle tyre sensors. Specifically, the present invention relates to an antenna configured to be exploited in a system of vehicle tyre sensors.
Description of the Related Art
The incorporation of electronic devices within pneumatic tyres is taking a greater importance in order to increase the safety of vehicles. Tyre electronics may include sensors and other components suitable for obtaining information regarding the behavior of a tyre, as well as various physical parameters thereof, such as for example temperature, pressure, number of tyre revolutions, vehicle speed, etc.
Such information may become useful in tyre monitoring and/or alarm systems.
Furthermore, active control/safety systems of the vehicle may be based on information sent from sensor devices included within the tyres.
Active safety systems use information about the external environment of a vehicle to change its behavior in pre-crash time period or during the crash event, with the ultimate goal of avoiding a crash altogether. Initially, active safety systems were primarily focused on improving the vehicle longitudinal motion dynamics, in particular, on more effective braking Anti-lock Braking Systems (ABS) and Traction Control (TC) systems. TC systems prevent the wheel from slipping while improving vehicle stability and steerability by maximizing the traction and lateral forces between the vehicle's tyre and the road. These systems were followed by more powerful vehicle stability control systems, e.g., Electronic Stability Program (ESP), Vehicle Stability Control (VSC), and Dynamic Stability Control (DSC). These latter systems use both brakes and engine torque to stabilize the vehicle in extreme handling situations by controlling the yaw motion. Active suspension systems are also an important part in vehicle active safety systems. They have been traditionally designed by trading-off three conflicting criteria: road holding, load carrying and passenger comfort. The suspension system has to support the vehicle, provide directional control during handling maneuvers and provide effective isolation of passengers/payload from road disturbances.
The active safety control systems described above are based upon the estimation of vehicle dynamics variables such as forces, load transfer, tire-road friction. The more accurate and “real time” the parameter estimation, the better the overall performance of the control system. Currently, most of these variables are indirectly estimated using on-board sensors, and are not very accurate. Using measurements made by sensors fitted on the vehicle tyres would provide far more accurate estimation of the parameters relevant to the vehicle dynamics.
Setting up a system based on sensors fitted on the vehicle tyres is however a challenging task, for several reasons.
The inside of a tire is a harsh environment experiencing high accelerations, and cannot be reached without taking the tire off the wheel. This situation poses very difficult problems: the high centrifugal acceleration implies that the sensor be light weight, for example not to unbalance the tyre, robust and small.
The fact that the tyre moves continuously with respect to the body of the vehicle forces to choose a wireless communication link for the communications from/to the sensors. However, the communication environment in which the sensor devices and the receiver are located is very harsh: in the immediate vicinity of a sensor device the wheel rim and the wheel arch of the car's body form two large signal reflectors. Both these parts are typically in metal and are curved in such a way that they tend to reflect incident waves back into the area, confining them. Furthermore, the radius of curvature of these two vehicle parts is of the order of the wavelength used for wireless transmission, making reflections much more complex. Also, the sensor device is inside the tyre and has to transmit through the tyre in some way: a true line of sight communication channel cannot be achieved since the tyre, being composed of a metal mesh and rubber, attenuates the signal dramatically.
Another issue is connected to the sensors' power supply; replacing the sensors' batteries is impractical because of the difficulty of reaching inside the tire. Hence, it is of primary importance that the sensor devices power consumption be as low as possible.
As disclosed in the U.S. patent application Ser. No. 12/654,705 filed on 29 Dec. 2009 and assigned to one of the present Applicants, herein incorporated by reference, some of the above issues can be solved by adopting a communication between sensor nodes fitted on vehicles' tyres and a sensor coordinator device fitted in the body of the vehicle exploiting Ultra Wide Band (UWB) transmission for the uplink (from the sensor nodes to the coordinator) and a narrowband transmission—such as one of the so called Industrial Scientific and Medical (ISM) radio bands—for the downlink (from the coordinator to the sensor nodes). The adoption of such communication scheme is advantageous because it allows to exploit the advantages of the UWB transmission for the uplink and at the same time the advantages of the ISM transmission for the downlink. Specifically, UWB is a technology suitable for low-cost, low-power, short-range and high-throughput wireless data transmission, which is robust against inter-symbol interference due to multi-path interference and lack of line-of-sight communications. Moreover, ISM transmission allows to strongly reduce the power consumption at the receiver side (sensor node) for the downlink, guaranteeing at the same time a sufficient throughput (which, in the downlink case, is relatively low).
Thus, by employing the solution proposed in such patent application, each sensor coordinator device, and each sensor node as well, need to be equipped with proper antennas, capable of transmitting and receiving both in the UWB band and in the ISM band.
Making reference in particular to the sensor coordinator devices, the system would be equipped with a pair of different antennas, one for receiving in the UWB band and one for transmitting in the ISM band. Said antennas should be carefully designed in such a way to fulfill the requirements imposed by the particular environment wherein they have to operate, such as being sufficiently compact and capable of exhibiting good performances, especially from the bandwidth point of view, even in presence of metallic elements included in the sensor coordinator devices themselves.
In order to reduce the area occupation of the antenna system, each coordinator device may be equipped with a single antenna, both for receiving in the UWB band and for transmitting in the ISM band; in this case, the antenna is referred to as “multiple-frequency antenna”.
Many types of multiple-frequency antennas can be found in the literature. Usually they are made with stacked patches on different layers, resonant ad different frequencies, or with complex shapes, including notches, slots, etc. To fulfill the compactness requirement, one of the most common patch antenna configurations is the planar inverted F antenna (PIFA), widely used in wireless terminals because of its small size for multiband applications. Examples of known multiple-frequency PIFAs are disclosed in “Thin internal GSM/DCS Patch Antenna for a Portable Mobile Terminal” by Kin-Lu Wong, Yuan-Chih Lin, and Ting-Chih Tseng, IEEE Transactions on antennas and propagation, vol. 54, No. 1, January 2006 and in “A compact PIFA suitable for Dual-Frequency 900/1800-MHz Operation” by Corbett R. Rowell and R. D. Murch, IEEE Transactions on antennas and propagation, vol. 46, No. 4, April 1998.
A further known PIFA is disclosed in “Regular circular and compact semicircular patch antennas with a T-probe feeding” by Y. X. Guo, K. M. Luk, and K. F. Lee, Microwave and optical technology letters, Vol. 31, No. 1., Oct. 5, 2001.